Method for manufacturing hot-formed steel product

ABSTRACT

A method of manufacturing a hot-formed steel product is provided, by which when a steel sheet is subjected to hot forming, excellent forming can be achieved without causing break or a crack during forming. When a steel sheet is subjected to hot forming to obtain a hot-formed product, the steel sheet is heated to be austenitized, then the steel sheet is cooled to a temperature range of martensite transformation start temperature Ms or less at an average cooling rate of 20° C./sec or more, and then the steel sheet obtained in this way is heated to Ac 1  transformation temperature or more, then subjected to hot forming, thereby an excellent hot-formed product is obtained.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a formed product in a field of manufacturing a sheet steel formed-product mainly used for a car body, in which a steel sheet (blank) as a material is heated to austenite+ferrite temperature (Ac₁ transformation temperature) or more, then the steel sheet is subjected to hot press-forming, so that a formed product is manufactured, and a formed product formed using such a steel sheet and the like, and particularly relates to a manufacturing method by which excellent forming can be achieved without causing break or a crack during press forming, a formed product and the like.

BACKGROUND ART

In car components, materials of the components are being increased in strength to achieve collision safety and weight saving together. Such components are typically manufactured by press-forming of a steel sheet. However, when cold working is applied to a steel sheet that was increased in strength, forming is hard, particularly in a material of more than 980 MPa.

From the facts, investigation is advanced on hot forming techniques for forming of a material steel sheet with the steel sheet being heated. As such a technique, for example, patent document 1 proposes a technique where a metal material is subjected to forming using a press mold at relatively low temperature while the metal material is heated at 850 to 1050° C. According to the technique, formability of a metal material is improved, in addition, occurrence of delayed fracture due to residual stress can be prevented. In particular, a component can be obtained, which has strength corresponding to that in a case that a high-strength steel sheet having a tensile strength of 1470 MPa class is used as a material, the steel sheet having been considered to be hard to be subjected to forming by a typical cold press method, and is excellent in dimension accuracy.

FIG. 1 is a schematic explanatory diagram showing a mold configuration for implementing hot forming (hereinafter, sometimes called “hot stamping”) as above, wherein 1 shows a punch, 2 shows a die, 3 shows a blank holder, 4 shows a steel sheet (blank), BHF shows blank hold force, rp shows punch shoulder radius, rd shows die shoulder radius, and CL shows clearance between the punch and the die respectively. Among the mold components, the punch 1 and the die 2 have channels 1 a, 2 a formed within them respectively, each of which may allow passing of a cooling medium (for example, water) through it, and the components are configured to be cooled by running the cooling medium through the channels.

In hot stamping (for example, hot deep drawing) using the mold as above, forming is started while a blank (steel sheet 4) is heated to Ac₃ transformation temperature or more and thus softened. That is, in a condition that the blank at high temperature is sandwiched between the die 2 and the blank holder 3, the steel sheet 4 is pressed into a hole of the die 2 by the punch 1, so that the steel sheet is formed into a shape corresponding to an outline of the punch 1 while the outer diameter of the blank is reduced. While reduction in temperature of the blank occurs by the punch and the die during forming, a material of the steel sheet is finally quenched by being kept to be cooled at a bottom dead center in forming. Such a forming method is implemented, thereby, for example, a component of 1470 MPa class having an excellent dimension accuracy can be obtained, in addition, since a forming load can be reduced compared with a case that a component in the same strength class is subjected to cold forming, a pressing machine can be reduced in capacity.

However, since the heated blank is different in contact timing to a mold depending on a region, temperature distribution occurs in the blank, therefore unevenness in material strength tends to occur due to the temperature distribution in the same blank. In particular, in deep-drawing forming requiring a blank holder, a blank portion to be held between the blank holder and the die is abruptly reduced in temperature during forming. Since deformation load of a material is increased in accordance with such temperature reduction, break or a crack tends to occur in the material during forming. From this, there is a problem that even if a condition that the blank is softened by heating is given, drawing forming cannot be performed because of the above reason.

As a steel sheet usable for hot stamping, for example, steel sheets as in patent documents 2 and 3 are proposed. The steel sheets are based on a technique that a chemical composition of a steel sheet is specified, thereby hardenability is improved after hot forming. While a steel sheet having improved hardenability after forming is obtained by the technique, break or a crack may still occur in the steel sheet in some shape of a component to be manufactured or hot-forming condition.

In the hot stamping technique as above, a blank is typically subjected to working after being heated to austenite temperature (Ac₃ transformation temperature) or more, it is proposed that a blank is heated to a temperature being supposed to be not more than the Ac₃ transformation temperature and subjected to press forming (for example, patent document 4). However, the inventor has found that when a blank is subjected to forming after being heated to the above temperature range, drawing formability tends to be further reduced (non-patent document 1).

[Patent Document 1]

JP-A-2002-102980, claims and the like.

[Patent Document 2]

JP-A-2004-124221, claims and the like.

[Patent Document 3]

JP-A-2004-315927, claims and the like.

[Patent Document 4]

JP-A-2003-126920, claims, and paragraph numbers [0041] and [0042].

[Non-Patent Document 1]

“Hot Stamping Drawability of Steel”, Proceedings IDDRG 2004, p 344.

DISCLOSURE OF THE INVENTION

The invention was made under such a circumstance, and an object of the invention is to provide a method useful for manufacturing a hot-formed steel product, by which when a steel sheet is subjected to hot forming, excellent forming can be achieved without causing break or a crack during forming, and a hot-formed product formed using the steel sheet, and the like.

The method of manufacturing the hot-formed steel product of the invention, by which the object was able to be achieved, has the gist in that a steel sheet, which is used for manufacturing a formed product through hot forming of the steel sheet, is heated to be austenitized, then the steel sheet is cooled to a temperature range of martensite transformation start temperature Ms or less at an average cooling rate of 20° C./sec or more, and then the steel sheet after cooling is subjected to hot forming.

The steel sheet obtained by the heat treatment is excellent in formability (particularly, drawing formability). Moreover, such a steel sheet is heated to a temperature of Ac₁ transformation temperature or more, then subjected to hot forming, thereby a hot-formed product having excellent quality is obtained.

ADVANTAGE OF THE INVENTION

In the invention, heat history is previously given, which is heat history that a steel sheet is heated to be austenitized, then the steel sheet is cooled to a temperature range of the martensite transformation start temperature Ms or less at an average cooling rate of 20° C./sec or more. Thus, a steel sheet for hot forming can be achieved, which is excellent in formability during hot forming, and easy to be subjected to drawing forming, and a formed product having excellent quality can be obtained without causing break or a crack during forming by using such a steel sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic explanatory diagram showing a mold configuration for implementing hot forming;

FIG. 2 is a schematic explanatory diagram showing a configuration of a previously developed mold;

FIG. 3 is photographs as a substitute for drawings showing structures of various steel sheets;

FIG. 4 is a bar graph showing results on a maximum forming load and presence of a crack when a test piece is subjected to forming;

FIG. 5 is a perspective view schematically showing an outward configuration of a formed product when forming was successfully performed; and

FIG. 6 is a perspective view schematically showing an outward configuration when a crack occurs.

DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS

-   -   1 punch     -   2 die     -   3 blank holder     -   4 steel sheet (blank)     -   7 pin

BEST MODE FOR CARRYING OUT THE INVENTION

The inventor has previously proceeded with study on a technique for achieving excellent press formability, and as a part of the study, the inventor proposed a technique of drawing forming using a mold as shown in FIG. 2 (JP-A-2005-14002). In a configuration of the die, pins 7 for supporting a steel sheet are provided on part of the blank holder 3, and the steel sheet 4 is set on the pins 7, so that the steel sheet can be close to the die 2 and the blank holder 3 without directly contacting to them (in FIG. 2, configurations of other portions are basically the same as those in the above FIG. 1). A top of the pin 7 is configured to be at the same level as that of the top of the blank holder during forming, so that the steel sheet 4 is set on the blank holder 3.

In the mold configuration as above, the steel sheet 4 is supported by the pins 7 so that the steel sheet 4 is avoided to be directly contacted to the mold (particularly, the die 2 and blank holder 3), thereby a top portion of the punch 1 and most of other portions are cooled approximately concurrently, consequently material strength on a punch surface and material strength on a flange surface are prevented from being relatively reduced due to unevenness in temperature of the steel sheet 4. As a result, break in the punch surface is particularly prevented, so that drawing formability is improved.

On the other hand, the inventor has further found that when oxidized scale having a predetermined thickness is designed to exist on a steel sheet surface, drawing formability is improved. That is, in usual hot forming, considering post-treatment after forming, heating is performed in a nonoxidative atmosphere in the light of preventing oxidation of a blank surface, and it has been considered that oxidized scale formed on the blank surface is preferably thin to the utmost (for example, 10 μm or less). However, according to investigation of the inventor, it was found that when oxidized scale was intentionally formed on a steel sheet surface, local reduction in temperature during forming is avoided, and therefore formability was rather improved, and since the inventor recognized technical significance of the finding, it separately made an application on it (Japanese Patent Application No. 2004-151753).

While the hot drawing formability of a steel sheet has been able to be remarkably improved by the techniques, a case that the techniques cannot be sufficiently utilized is supposed in some situation.

Thus, the inventor made further investigation in order to improve deformation ability of a material steel sheet itself from a point of a structure, as a result, the inventor found that the heat history as above was previously given to the steel sheet, thereby formability was excellent during hot forming and therefore drawing forming was easily performed, and consequently completed the invention.

A steel sheet used for the hot-formed steel product of the invention is obtained by heating the steel sheet (to Ac₃ transformation temperature or more) to be austenitized, then cooling the steel sheet to a temperature range of the martensite transformation start temperature Ms or less at an average cooling rate of 20° C./sec or more. The steel sheet is further improved in hot formability by giving such heat history to the steel sheet.

The reason for setting heating temperature of the steel sheet to be the Ac₃ transformation temperature (austenitizing temperature) or more is to dissolve carbides in the steel sheet in austenite. The reason why the steel sheet is cooled to the martensite transformation start temperature Ms or less at an average cooling rate of 20° C./sec (hereinafter described as “° C./s”) or more is because it is necessary for making a microstructure of a steel sheet obtained such a series of cooling to be a uniform martensite structure to the utmost, and when the average cooling rate is less than 20° C./s, since the amount of a ferrite structure, a bainite structure and the like are increased in a martensite structure after cooling, hot drawing formability is not expected to be improved. Therefore, the average cooling rate is preferably 50° C./s or more.

While the reason for improvement in formability by the steel sheet used in the invention is not wholly clarified, it may be considered as follows. That is, the blank is once austenitized, thereby carbides that have been present in the steel sheet are disappeared (dissolved in austenite), and then the blank is rapidly cooled, so that a microstructure of the steel sheet is homogenized. When the steel sheet in such a state is reheated (heating before forming) before forming, since a carbide that tends to be a start point of fracture does not exist in the microstructure of the steel sheet during forming, fracture limit is expected to be increased.

In the steel sheet that was subjected to heat history that the steel sheet was austenitized and then rapidly cooled, when the steel sheet is heated to a range of a dual phase region, a microstructure during forming shows a lath structure. It is considered that such a phenomenon may be a factor of decrease in forming load or improvement in fracture limit, leading to further improvement in formability.

FIG. 3 (photographs as a substitute for drawings) shows structures of various steel sheets. FIG. 3( a) shows a structure of a steel sheet (steel sheet of the invention) that was subjected to the heat history, and FIG. 3( b) shows a structure of a steel sheet (usual steel sheet) that was not subjected to the heat history. In the figures, whitely photographed portions correspond to martensite, and blackly photographed portions correspond to ferrite, and it is known that a structure of the steel sheet used in the invention shows a lath structure.

Generally, it is considered that the lath structure does not disappear due to hot working. The reason for this is considered as follow: since a steel sheet is cooled by a mold immediately after hot forming, a structure during heating or forming is frozen with being substantially not changed.

In a manufacturing method of the invention, when a steel sheet is subjected to hot forming, the sheet is heated to the Ac₁ transformation temperature or more (austenite+ferrite range). As forming temperature is closer to the heating temperature (that is, cooling time from heating to forming start is shorter), a more desirable result is obtained (see FIG. 4 described later). This is because as cooling time is longer, a larger part of austenite, which was formed during heating, is decomposed, consequently hardness becomes insufficient against an object of hot stamping. Moreover, a preferable range of the cooling time is different depending on sheet thickness, and as sheet thickness becomes larger, a smaller effect is given even in long-time cooling. For example, when a steel sheet 1.4 mm in thickness is cooled, cooling time is preferably within 20 sec.

While an advantage of the manufacturing method of the invention is significantly exhibited in the case that a steel sheet is subjected to forming using a mold having a blank holder (that is, drawing forming), it is also useful that the previously proposed technique is further used together with meeting such a requirement. That is, it is also useful that the mold configuration as shown in FIG. 2 is used to achieve evenness in temperature of a steel sheet, or press forming is performed using a steel sheet of which the surface has oxidized scale 15 μm or more in thickness formed thereon. By using such techniques together, the advantage of the invention is more effectively exhibited.

Moreover, as clear from the above meaning, a formed product according to the invention includes not only a drawing-formed product obtained by forming using the blank holder, but also a product obtained by usual press forming. Even if such a formed product is manufactured, an advantage according to the manufacturing method of the invention is achieved.

A chemical composition of the steel sheet used in the invention is not particularly limited, and a steel sheet typically used for hot press can be used. Desirably, a steel sheet can be used, which contains 0.10 to 0.35 mass % C, 2 mass % or less Mn, 0.1 to 3.0 mass % Si, 0.01 to 0.5 mass % Al, 0.001 to 0.05 mass % Ti, 0.005 mass % or less B, 0.03 mass % or less P, and 0.02 mass % or less S respectively, and the remainder including Fe and inevitable impurities, and may be further contain 0.01 to 1 mass % Cr, 0.01 to 1 mass % Me, and 0.005 to mass % Nb as necessary. The heat history may be given to a steel sheet immediately before hot press, however, even in the case of a steel sheet that is previously subjected to such a heat history and then left for a certain period, the advantage of the invention can be exhibited.

Hereinafter, the advantage of the invention is further specifically shown according to an example. However, the following example is not intended to limit the invention, and any design change based on the meaning described before and later is included in the technical scope of the invention.

EXAMPLE

Steel having a chemical composition as shown in the following Table 1 was rolled into a thickness of 1.4 mm by typical means and then annealed. Then, circular blanks having a diameter (blank diameter) of 95 mm were punched from the rolled steel and used for an experiment (the blank has Ac₁ transformation temperature of 725° C., Ac₃ transformation temperature of 850° C., and Ms temperature of 450° C.). For the circular blanks, the following various test pieces were prepared.

Test piece A: the circular blank being not subjected to heat treatment (subjected to only rolling and annealing as above: usual material),

Test piece B: the circular blank being heated to a temperature of 900° C. to be austenitized, then cooled to 300° C. by water cooling (average cooling rate of 20° C./s) (material of the invention), and

Test piece C: the circular blank being heated to a temperature of 900° C. to be austenitized, then cooled to 300° C. by forced air cooling (average cooling rate of 10° C./s) (comparative material).

TABLE 1 Chemical composition of blank (mass percent) C Si Mn Cr B remainder 0.23 0.19 1.22 0.24 0.0019 Fe

Each of the circular blanks after heat treatment (test pieces A to C) was subjected to hot square-cylinder drawing forming using a mold (square cylinder die and square cylinder punch) with a head profile of the punch being a square (one side is 45 mm long) (see the above FIG. 2). At that time, the blank was heated in air atmosphere using an electric furnace, and heating temperature was set at 770° C. or 810° C. Moreover, heating holding time was controlled for each heating temperature during heating, thereby thickness of oxidized scale formed during heating was uniformed to be about 20 μm.

The forming experiment was performed using the mold shown in the above FIG. 2 which was installed in a crank press machine. Forming speed was set to be 40 times/min in crank rotational speed. Forming start temperature was controlled by changing time (cooling time) before starting forming after taking out a blank from a heating furnace. At that time, the forming start temperature was estimated from the cooling time before forming start (5 sec or 10 sec) based on a natural cooling curve of a blank being previously measured. In a forming process, after forming is started, a steel sheet was held for about 20 sec at a bottom dead center, so that the steel sheet was subjected to quenching operation. Other press forming conditions are as follows.

Other Press Forming Conditions

Blank hold force: 3 tons,

Die shoulder radius rd: 5 mm,

Punch shoulder radius rp: 5 mm,

Clearance between punch and die CL: [1.32/2+1.4 (steel sheet thickness)] mm,

Forming height: 37 mm,

Lubricant: calcium oxide based paste-like solid lubricant was used, and coated on the mold.

FIG. 4 (bar graph) shows results on a maximum forming load and presence of a crack when each test piece is subjected to forming. At that time, the maximum forming load means a maximum load necessary in forming, and as the load is smaller, better formability is shown.

In FIG. 4, a sign “◯” shows that forming can be performed without occurrence of any crack, and a sign “x” means that a crack occurs during forming. A white bar shows a result when heating temperature is 770° C., and a shaded (hatched) bar shows a result when the heating temperature is 810° C.

When the heating temperature was 770° C., the forming start temperature (estimated temperature) was 725° C. (cooling for 5 sec) and 680° C. (cooling for 10 sec), and when the heating temperature was 810° C., the forming start temperature was 755° C. (cooling for 5 sec) and 705° C. (cooling for 10 sec), respectively.

As clear from the results of FIG. 4, it is known that in a case of the method of the invention (using the test piece B), the maximum forming load is low, and a desired formed-product is obtained without occurrence of a crack. On the contrary, it is known that in cases of the test pieces A and C, the maximum forming load is increased, and a crack tends to occur.

FIG. 5 schematically shows an outward configuration of a formed product obtained by the above (in which no crack occurs). FIG. 6 schematically shows an outward configuration of a formed product in which a crack occurs.

On a relationship between the cooling time and the maximum forming load, a tendency was shown, which is tendency that as the cooling time is increased, the maximum forming load is increased. This was able to be considered because strength of a steel sheet was increased, leading to increase in deformation resistance of the steel sheet. 

1. A method of manufacturing a hot-formed steel product, comprising the following steps: heating a steel sheet to be austenitized, cooling the steel sheet to a temperature range of martensite transformation start temperature Ms or less at an average cooling rate of 20° C./sec or more, and performing hot forming of the steel sheet after cooling to obtain a hot-formed steel product.
 2. The method of manufacturing the hot-formed steel product according to claim 1: wherein the steel sheet that has been cooled is heated to Ac₁ transformation temperature or more, then subjected to hot forming.
 3. The method of manufacturing the hot-formed steel product according to claim 1: wherein the steel sheet contains 0.10 to 0.35 mass % C, 2 mass % or less Mn, 0.1 to 3.0 mass % Si, 0.01 to 0.5 mass % Al, 0.001 to 0.05 mass % Ti, 0.005 mass % or less B, 0.03 mass % or less P, and 0.02 mass % or less S respectively. 